A bearing is a mechanical element that permits a shaft of a machine to rotate, fixes the rotating shaft in position, and supports the unladen weight of the shaft and a load applied to the shaft. When a rotating machine makes a number of turns per unit time, repetitive load in proportion to the turns is applied to a bearing that supports a rotating shaft. Since the repetitive load is applied, the bearing is required to have high resistance against fatigue fracture caused by the repetitive load as well as excellent abrasion resistance. A process of manufacturing the bearing includes the steps of properly controlling the composition of steel, which will be made into the bearing, forming a steel wire rod through steel-making, continuous casting and rolling, and machining the resultant steel wire rod into the bearings.
For the manufacturing of bearings, high-carbon chromium steel containing about 1.0 wt % carbon (C) and about 1.5 wt % chromium (Cr) has been mist widely used.
In general, the high-carbon Cr bearing steel is frequently manufactured into a bloom through converter, vacuum degassing (RH: Rheinstahl huttenwerke & Heraus) and continuous casting processes. The converter process includes a blowing process of converting molten iron into molten steel, and the vacuum degassing is carried out so as to decrease, as much as possible, total oxygen content that indicates inclusion content in the vacuum degassing process. The resultant bloom is subjected to soaking for relieving segregation and removing coarse carbide from the central portion of a bloom, followed by natural air cooling, and is then rolled into a billet, which is in turn formed into a wire rod via rolling.
A bearing is manufactured by machining the resultant wire rod into the shape of a transmitter of the bearing, such as balls or inner/outer wheels, through drawing and spheroidizing heat treatment, followed by hardening heat treatment for securing sufficient fatigue strength and abrasion resistance.
The typical hardening heat treatment on the bearing is carried out by a so-called QT process in which quenching is followed by tempering. The quenching rapidly cools down the bearing after heated to or above Ac3 temperature where steel is austenitized. By the quenching, the internal structure of steel is converted from austenite into hard martensite via diffusionless transformation. Although the martensite produced by the quenching is very hard, it cannot be directly used as the bearing due to poor toughness. Hence, the quenching is followed by the tempering for improving the toughness of the bearing. The tempering is a precipitation process for the martensite formed by the diffusionless transformation and is carried out by maintaining the bearing at a predetermined temperature such that carbon supersaturated in the martensite can precipitate as carbide so as to alleviate or remove residual stress caused by the quenching and to anneal hardened microstructures. As important features of the tempering, fine carbide precipitates in the martensite, and the martensite is converted into tempered martensite.
When produced by the above-described process, bearing steel has an internal structure composed of tempered martensite and carbide, and its surface hardness is about 60HRC or higher. Even if a bearing is heat-treated by the above QT process, its abrasion resistance and fatigue resistance are frequently insufficient when used in harsh conditions such as rolling contact.
This is because carbide formed in the tempered martensite has an adverse effect on the toughness of steel. This occurs when the internal structure of steel is formed close to 100% tempered martensite. Even if hardness is very high as described above, toughness and ductility are poor. Accordingly, the problem is that toughness and ductility are not sufficiently improved in the tempered martensite.
Furthermore, the QT process requiring at least 2 hours causes other problems such as increase in manufacturing costs and decrease in productivity.
The present invention has been made to solve the foregoing problems with the prior art. According to one or name aspects of the present invention, there is provided a steel wire rod for bearing steel, which can be advantageously manufactured into novel bearing steel by quenching and partitioning, such that the novel bearing steel has greatly improved toughness over those manufactured by conventional QT treatment.
According to one or more aspects of the present invention, there are provided a heat treatment method of novel bearing steel, which can have greatly improved toughness even with a shorter heat treatment time compared to conventional QT treatment, and a bearing manufactured by the same method.
According to one or more aspects of the present invention, there is provided a steel wire rod for bearings, which can be advantageously used for the manufacturing of the above-described bearing of the present invention.
According to one or more aspects of the present invention, there is provided a soaking method of a steel bloom, by which segregation in the steel bloom can be removed so as to advantageously provide the above-described bearing of the present invention.